Tin modified ferromagnetic chromium oxide, its preparation and use in recording members



United States Patent TIN MODIFIED FERROMAGNETIC CHROMIUM OXIDE, ITSPREPARATION AND USE IN RE- CORDING MEMBERS John N. Ingraham, Wilmington,Del.,

du Pont de Nemours and Company a corporation of Delaware I No Drawing.Application June 19, 1957 Serial No. 666,745

9 Claims- (Cl. 252-625) assignor to E. I. Wilmington, Del.,

This invention relates to ferromagnetic materials. More particularly,:-it relates to a 'new-..-ty'pe -:of ferromagnetic chromium oxide and toamethodsfor itsrpreparation. The invention also relates to magneticrecording members having :this new type of ferromagnetic chromium oxideas theimagriet'iccomponent. i t

Ferromagnetic materialsg'are employed in avariety of applications. For''example, they are used inmagnetic sound recording tapesjdr'ums "andrecords, memory devices, microwave circuitry, and as 'r'nagnetic cores,such as coil cores in electronic equipment. In some of theseapplications, especially those requiring magnetic materials of low losscharacteristics at high frequencies, or of relatively high coercivity,ferromagnetic oxides are normally more useful than ferromagnetic metals.

Heretofore, ferromagnetic iron oxides have been used extensively in themanufacture of magnetic recording tapes. Recently, some novel types offerromagnetic chromium oxides have been developed which possess certainmagnetic properties which make them of use in various applications. Onesuch product, which is fully described and claimed in United Statesapplication Serial No. 617,283, filed October 22, 1956 by Alfred L.Oppegard, is a ruthenium-modified chromium oxide. It has a particularcombination of magnetic properties, i.e., the combination of highintrinsic coercive force, H and high specific magnetization, as, thatmakes these particular chromium oxides especially useful as the magneticcomponent in magnetic recording tapes. While these ruthenium-modifiedchromium oxides are capable of producing superior magnetic recordingtapes, the high cost of the ruthenium dioxide used in preparing themmakes the process relatively expensive. It is, therefore, a desirablegoal to develop new methods of making ferromagnetic chromium oxideshaving a desirable combination of magnetic properties by a moreeconomical process. It is also desirable to provide ferromagneticchromium oxides having a specific combination of magnetic propertieswhich make individual products useful in specific applications.

It is an object of this invention to provide a new form I offerromagnetic chromium oxide and a method for its preparation. A furtherobject is to provide a new form of ferromagnetic chromium oxide which isespecially useful in the manufacture of magnetic recording tapes and inthe manufacture of ceramic bodies such as magnetic cores. Another objectis to provide a magnetic recording member having a magnetic trackcomprising this new form of ferromagnetic chromium oxide. Other objectswill appear hereinafter.

These and other objects of this invention are accomplished by providingferromagnetic chromium oxides containing 55.0-61.5% chromium and0.4-5.0% tin, and having a tetragonal crystal structure. Thetin-modified chromium oxides of this invention are produced in a formwhich consists essentially of acicular particles having lengths in therange of about 0.2-1.0 micron with an axial ratio, i.e., a ratio oflength to width, within the 2,923,684 Patented Feb. 2, 1960 ice fromabout 50 to about 85 electromagnetic units/gram (E.M.U./g.)' with thepreferred products having specific I magnetizations ranging between 64and 85 E.M.U./g.

The products of this invention have a Curie temperature of about 123 C.

In view of the magnetic and physical properties of the tin-modifiedchromium oxides of this invention, they are especially useful asrecording members. includes magnetic recording members which comprise acarrier, such as a tape, drum, or record, of nonmagnetic material havingbonded thereto a magnetic track of the ferromagnetic chromium oxides ofthis invention and a binder therefor.

The novel ferromagnetic chromium oxides of this invention are preparedby the novel process which comprises heating chromium trioxide, CrO at atemperaturewithin the range of 375-475" C. under a pressure of at least200 atmospheres in the presence of water and from 0.75-7.5% or more,based on the weight of chromium trioxide, of a tin compound of the groupconsisting of stannous sulfate and stannic sulfide and separating anddrying the resulting tin-modified ferromagnetic chromium oxide of highcoercivity and specific magnetization.

The use of a small amount of an alkali metal sulfate, e.g., sodium,potassium, or lithium sulfate, or sulfuric acid or cadmium sulfate inconjunction with the tin compound in the process of this invention isbeneficialin that such additives improve the magnetic properties of theproducts in comparison with the properties obtained by the use ofstannous sulfate or stannic sulfide alone. The preparation offerromagnetic chromium oxide from chromium trioxide in the presence ofstannous sulfate orstannic sulfide, together with an alkali metalsulfate, sulfuric acid, or cadmium sulfate, is a preferred optionalembodiment of the invention. Ferromagnetic chromium oxides having thebest combination of magnetic properties for use in magnetic recordingtapes are provided by this preferred embodiment.

The process of this invention can be carried out in acorrosion-resistant container, i.e., a container constructed of amaterial which is inert to the reactants under the reaction conditions.Satisfactory containers are made of platinum or of the alloy knowncommercially as Hastelloy" C. The container can be a completely sealedtube having flexible walls or it can be any type of vessel permittingtransmission of the desired pressure to the reaction system, such as acylindrical tube closed at one end and equipped at the other end with aclosely fitted piston, or it can be a pressure-resistant vessel having acorrosion-resistant lining.

The process is conveniently carried out in a container of the type thatis placed inside a larger pressure vessel which is used to apply thepressure to the corrosionresistant vessel containing the reactants. 'Thecontainer is charged with chromium trioxide, 0.75-7.5%, preferably1.0-5.0% of stannous sulfate or stannic sulfide, based on the Weight ofchromium trioxide, and from 0.1-6.0 parts of water for each part ofchromium trioxide. Preferably an amount of water ranging from 10-50% ofthe Weight of the chromium trioxide is used. When the preferredembodiment of the process is being carried out, there is also added tothe container up to 5%, preferably 1-5% of the weight of the chromiumtrioxide, of an alkali metal sulfate, sulfuric acid, or cadmium sulfate.

The corrosion-resistant container is closed'and placed They have anintrinsic coercive the magnetic material inmagnetic; Accordingly, thepresent invention inside a larger vessel capable of withstanding hightemperatures and pressures. The outer vessel is then closed andpressured with an inert liquid or gas, e.g., water or helium, so that atthe operating temperature the pressure is at least 200 atmospheres. Goodresults are obtained with pressures ranging from 2004000 atmospheres.Pressures of 3000 atmospheres and above can be used if the availableequipment is strong enough to withstand them. The reaction vessel isthen heated to a temperature between 3-75 and 475 C. and preferablybetween 400 and 450 C.

The reaction times are not critical. Periods ranging from a few minutes,e.g., five minutes, to three hours or more at the reaction temperatureof 375-475 C. can be used. Reaction times of 1-3 hours give satisfactoryresults. Likewise, the time of heating the reaction vessel to thereaction temperature and the time of cooling it back after reaction iscompleted can vary considerably, depending on the size of the vessel andthe charge and on the capacity of the heating and cooling equipment.However, it is preferred that heating to reaction temperature becompleted in less than one hour and that cooling be completed in 24hours.

After the reaction mixture has been heated to the desired operatingtemperature for the desired time, the entire reaction vessel is cooledwith the pressure 'maim tained until room temperature, about 25 C., isreached; The pressure is then released and, if a sealedcorrosionresistant vessel is employed, this may result in it being 7ruptured by the by-product oxygen present in the container. Theresultant, slightly compacted or aggregated, black, acicular chromiumoxide is separated from the dark-colored aqueous phase by filtration andis then washed and dried. A convenient way of doing this is to break upthe lumps of crude reaction products in a mortar. This treatment merelybreaks up the aggregates; it does not reduce the size or change theshape of the individual particles. The finely divided product is thenwashed by immersing it in distilled water and heating the water toboiling, followed by decantation or filtration. The washingin boilingdistilled Water is repeated two more times and then the product iscollected on a filter, rinsed thoroughly with distilled water, and driedat about 150 C. for 60-65 hours in a vacuum of about 2530 inches ofmercury.

Tin is contained in the ferromagnetic chromium oxides of this invention,as evidenced by the inability to recover tin from the products bywashing, magnetic separation or by other methods. Furthermore, since thecrystal structure of tin oxide is similar to that of the chromium oxidesof this invention, it is believed that the tin is present in positionsin the crystal lattice normally occupied by chromium atoms. In any case,all the high coercive force chromium oxides of this invention containsmall amounts, i.e., from 0.4-5.0% of tin.

Similarly, when the chromium oxides of this invention are made by thealternative embodiment in which an alkali metal sulfate or cadmiumsulfate is included in the reaction mixture, the ferromagnetic chromiumoxide obtained also contains a small amount, i.e., up to 1%, of thealkali metal or cadmium.

. Chromium trioxide, stannous sulfate, stannic sulfide, alkali metalsulfate, sulfuric acid and cadmium sulfate of the grades commerciallyavailable are suitable for use in the process of this invention.

As indicated above, the ferromagnetic tin-modified chromium oxides ofthis invention possess a number of properties, or characteristics, whichmake them especially suitable for use in certain application. Inaddition to their being of very small particle size, i.e., havinglengths between about 0.2 and 1.0 micron, the crystals are acicular inshape and are believed to be single crystals. On the basis oftheoretical calculations, the particle size is about that expected forsingle domain particles. Single domain particles are especially llSfifulin certain magnetic applications. Their acicular shape also makes themespecially well suited for use in coating compositions applied on films,tapes, or other substrates. As a result of the acicular shape, theparticles can be oriented in closer relationship during the mechanicalspreading of the oxide composition in thin layers on a substrate. Thisin turn results in more uniform magnetic characteristics of the coatedsubstrate.

The tetragonal crystal structure of the chromium oxides of thisinvention is of the rutile type, i.e., it has the same type of crystalstructure as rutile, TiO- Ferromagnetic chromium oxides prepared byhitherto known methods have also exhibited the tetragonal crystalstructure, but in all oxides other than those recently developed byapplicants assignee such as, for instance, the oxides of theaforementioned Oppegard application, other crystal structures havealways been associated with the rutile-type structure, e.g., thecorundum-type hexagonal crystal structure of Cr O In contrast, theferromagnetic chromium oxides of this invention have only a singlecrystal structure as shown by X-ray diffraction and this is thetetragonal crystal structure of the rutile type.

The ferromagnetic tin-modified chromium oxides of this invention exhibitseveral magnetic characteristics which make them especially valuable foruse in various applications. These particular properties, whichv havebeen mentioned previously, and which are critical factors in theirusefulness in certain magnetic applications, are the intrinsic coerciveforce, Hci, and specific magnetization of sigma, as. The definition ofthe intrinsic coercive force, H is given in Special TechnicalPublication No. of the American Society for Testing Materials, entitledSymposium on Magnetic Testing (1948), pp. 191498. The values for theintrinsic coercive force given herein are determined on a DC.ballistic-type apparatus which is a modified form of the apparatusdescribed by Davis and Hartenheim in the Review of ScientificInstruments 7, 147 (1936). The sigma value, as, is defined on pages 7and 8 of Bozorths Ferromagnetism, D. Van Nostrand Co., New York, 1951.This sigma value is equal to the intensity of magnetization, 1,, dividedby the density, d, of the material. The sigma values given herein aredetermined on apparatus similar to that described by T. R. Bardell onpp. 226-228 of Magnetic Materials in the Electric Industry,Philosophical Library, New York, 1955.

The tin-modified ferromagnetic chromium oxides of this invention havingintrinsic coerctive forces, Her of more than 140, and especially thosehaving values of 170-260 oersteds, and having sigma values, as, of 5085,and especially 64-85 E.M.U./g., are particularly suitable for use as themagnetic component in magnetic recording tapes.

The invention is illustrated further by the following examples in whichthe proportions of ingredients are expressed in parts by weight unlessotherwise noted.

EXAMPLE I A flexible platinum tube is charged with 5.0 parts of chromiumtrioxide, 0.05 part of stannous sulfate, and 0.85 part of water. in awater-filled pressure vessel and heated to 400 C. under a pressure of7307 50 atmospheres for three hours. The pressure is maintained as thereaction vessel is cooled to room temperature and, after depressuring,the platinum tube is removed and carefully opened. The product isremoved, any lumps broken up in an agate mortar and then washed byplacing in distilled water and heating the water to boiling. Thiswashing is repeated two more times and the washed product is collectedon a filter, rinsed thoroughly with distilled water, and dried in avacuum oven at C. under a vacuum of 2530 inches of mercury for 65 hours.The product, a tinmodified ferromagnetic chromium oxide, is a stronglymagnetic, black powder having an intrinsic coercive The tube is sealedand then placed force, H of 142 oersteds and a saturation magnetization,as, of 82.3 E.M.U./ g.

EXAMPLE II A mixture of 3.0 parts of chromium trioxide, 0.06 part ofstannous sulfate, 0.05 part of potassium sulfate, and 0.46 part of wateris heated in a sealed platinum tube as described in the precedingexample. A temperature of 398-400 C. at a pressure of 750-780atmospheres is maintained for 3 hours. The product, isolated asdescribed in Example I, is a black, strongly magnetic solid having anintrinsic coercive force of 198 oersteds and a saturation magnetizationof 64.0 E.M.U./g.

When the process of Example II is repeated with the exceptionthatidentical quantities of lithium sulfate and sodium sulfatearesubstituted;for the potassium sulfate, ferromagnetic products havingcoercive forces of 170 and 158 oresteds, respectively,-and sigma valuesof 63.8 and 65.5 E.M.U./g.,,srespectively, are obtained.

EXAMPLE-III AnaIysis.Cr, 58.76, 58.85%; Sn (as SnO 1.15%; K, 0.37%; H1.81, 1.21, 1.30%.

The product shows only the X-ray diffraction pattern characteristic ofchromium dioxide.

EXAMPLE IV The tin-modified ferromagnetic chromium oxide of Example IIIis employed in the preparation of a'magnetic recording tape as follows:Ten parts of this ferromag netic chromium oxide is dispersed in 35 partsof a 1:1 toluene/tertiary butyl alcohol mixture containing 0.3 part ofcetyldimethylamine by milling for two days in an S-ounce glass jarcontaining 160 parts of one-quarter inch glass beads. To the resultingdispersion there is added l6.6 parts of a 20% solution of polyvinylbutyral in 1:1 toluene/tertiary butyl alcohol mixture containing 0.5part of dibutyl sebacate. Milling is continued for an additional twodays. The dispersion is filtered and coated onto a l-mil polyethyleneterephthalate film by means of a doctor knife set at 5 mils clearance.The coated film is dried, leaving a coating 0.5 mil in thickness.

a The tape prepared as described above is tested for performance in amagnetic tape recorder (Ampex No. 307") at a tape speed of 30 inches persecond with optimum bias current. The tape is subjected to a constantsound input at several frequencies between 1 and 20 kilocycles. Theunequalized output signals at these different frequencies are thenmeasured, and the results are tabulated below.

Output at various frequencies Frequency (kc) EXAMPLE V A flexibleplatinum tube is charged with 3.0 parts of chromium trioxide, 0.06 partof stannous sulfate, and

0.58 part of 20% sulfuric acid. The tube is sealed and then heated at atemperature of 395-403 C. under a pressure of 750-785 atmospheres forthree hours. After cooling the reaction vessel and isolating the productas in the previous examples, there is obtained a black, stronglymagnetic solid having an intrinsic coercive force of 225 oersteds and asigma value, as, of 64.1 E.M.U./g. This ferromagnetic chromium oxidecontains 58.7% Cr and 1.05% Sn (on a bone-dry basis);

EXAMPLE VI A flexible platinum tube is charged with 30.0 parts ofchromium trioxide, 1.50 parts of stannous sulfate, 1.1 parts ofpotassium sulfate, and 4.0 parts of water, and the sealed tube is heatedat 445-459 C. under 990 atmospheres for three hours. After cooling thereaction vessel and isolating the product as in the preceding examples,there is obtained a black, magnetic solid having an intrinsic coerciveforce of 242 oersteds and a sigma value, as, of 72.1 E.M.U./g. Thisferromagnetic chromium oxide contains 57.0% chromium, 3.36% tin and0.14% potassium (on a bone-dry basis). The product shows the rutile-typeX-ray pattern characteristic of chromium dioxide.

EXAMPLE VII A mixture of 3.0 parts of chromium trioxide, 0.03 part ofstannous sulfate, 0.11 part of cadmium sulfate hydrate (3CdSO .8H O),and 0.40 part of water is heated as described in the preceding examplesin a sealed platinum tube at 396402 C. under a pressure of 720-780atmospheres for three hours. The black ferromagnetic chromium oxideobtained has a coercive force of oersteds and a sigma value, as, of 73.8E.M.U./ g. Analysis shows 60.4% chromium, and 0.049% cadmium (on abone-dry basis). X-ray fluorescence measurements indicate the presenceof 0.5-0.7% tin (calculated as SnO The X-ray diffraction pattern agreeswith that of chromium dioxide.

EXAMPLE VIII A mixture of 3.0 parts of chromium trioxide, 0.03 part ofstannic sulfide, and 0.51 part of water is heated in a sealed platinumtube as in the preceding examples at 400 C. under a pressure of 600745atmospheres for three hours. The product isolated as inthe precedingexamples is a ferromagnetic solid having an intrinsic coercive force of186 oersteds and a sigma value, as, of 72.5 E.M.U./g. This ferromagneticchromium oxide contains 60.2% chromium and 0.80% tin (on a bonedrybasis). The X-ray diffraction pattern is that of chromium dioxide.

An importantadvantage of the process of this invention over the best ofthe hitherto known methods of making ferromagnetic chromium oxides ofhigh coercivity and specific magnetization resides in its being a lowercost process. Stannous sulfate and stannic sulfide are considerably lessexpensive than ruthenium dioxide. Another advantage is that the highwater solubility of stannous sulfate permits a more uniform mixing ofthis sulfate throughout the reaction mixture.

While the ferromagnetic tin-modified chromium oxides of this inventionare especially valuable for use in magnetic recording tapes, drums, andrecords, they are also useful in other applications, especially in themanufacture of ceramic bodies, such as magnetic cores, when they arecompacted into shaped objects having about the theoretical density. Inthe compacted form, they are useful in magnetic memory cores forcomputers, in gymtor elements, in electrically operated high-frequencyswitches, and in low-loss transformer cores for magacycle/secondfrequency ranges. The tin-modified chromium oxides of this invention arealso useful as focusing magnets and in microwave attenuators.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it inventionis not limited to the specific embodiments thereof except as defined inthe appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A' ferromagnetic chromium oxide containing, by weight, 55.0% to'61.5% chromium and 0.4 to.5.0% tin, said chromium and tin being combinedwith oxygen, and said oxide having'a tetragonal crystal structure oftherutile type. p

2. A ferromagnetic chromium oxide containing, by weight, 55.0% to 61.5%chromium and 0.4 to 5.0% tin, said chromium and tin being combined withoxygen, said oxide having an intrinsic coercive force above 140 oerstedsand consisting essentially, of acicular particles of tetragonal crystalstructure having lengths in the range of about 0.2 to 1.0 micron, andhaving an axial ratio of from :1 to,l5:1.

3. A ferromagnetic chromium oxide as set forth in claim 2 in which saidchromium oxide has an intrinsic coercive force of between 170 and 250oersteds.

4. A magnetic recording member which comprises a carrier of non-magneticmaterial having bonded thereto a ferromagnetic chromium oxide as setforth in claim 2, and a binder therefor. v

5. A magnetic recording member-which comprises a carrier of non-magneticmaterial having bonded thereto a ferromagnetic chromium oxide as setforth in claim 3 and a binder therefor.

6. A ferromagnetic chromium oxide as set forth in claim 2 in which saidchromium oxide has a specific magnetization within the range of 50 to 85electromagnetic units/ gram.

7. Process for preparing a ferromagnetic chromium oxide of high coerciveforce which comprises heating at a temperature within the range of 375to 475 C. and under a pressure of at least 200 atmospheres chromium isto be understood that this assesses trioxide in the presence of waterand from 0.75% to 7.5% by weight based on said chromium trioxide of atin compound selected from the group consisting of stannous sulfate andstannic sulfide, and separating and drying as the resulting product aferromagnetic chromium oxide of high coercivity and specificmagnetization.

8. Process for preparing a ferromagnetic chromium oxide of high coerciveforce which comprises heating at a temper'autre within the range of 375to 475 C. and under a pressure of 200 to 3000 atmospheres, chromiumtrioxide, from 0.1 to 6.0 partsof water per part of chromium trioxide,and from 0.75 to 7.5 by weight based on said chromium trioxide of a tincompound selected from the group consisting of stannous sulfate andstannic sulfide, and separating and drying as the resulting product aferromagnetic chromium oxide of high coercivity and specificmagnetization.

9. Process as set forth in claim 7 in which up to 5%, based on theweight of chromium trioxide, of a com pound selected from the groupconsisting of an alkali metal sulfate, sulfuric acid and cadmium sulfateis present in the reaction system, and from 0.1 to 6.0 parts of waterbeing present per part of chromium trioxide.

Verway et al.: I. of Chemical Physics, vol. 15, pp. 174180, April 1947.

Kordes et al.: Chemical Abstracts, vol. 46, col. 4411,

' May 25, 1952.

1. A FERROMAGNETIC CHROMIUM OXIDE CONTAINING, BY WEIGHT, 55.0% TO 61.5%CHROMIUM AND 0.4 TO 5.0% TIN, SAID CHROMIUM AND TIN BEING COMBINED WITHOXYGEN, AND SAID OXIDE HAVING A TETRAGONAL CRYSTAL STRUCTURE OF THERETILE TYPE.